1. Field of the Invention
The present invention relates to an apparatus for processing a still screen in a digital video reproducing system, in which an appearance of a color pattern at a color boundary portion is eliminated when digital video data reproduced by a digital video reproducing system is displayed as a still screen mode.
2. Description of the Prior Art
Currently, a disc type information recording medium is widespread as an audio/video information recording medium on which a large amount of information is recorded. Accordingly, the disc type information recording medium has been developed on which video signals such as graphic picture data, a still picture and fully moving picture information of a movie program are recorded along with related audio signals. As the disc type information recording medium has the merit of being suitable for having a large amount of information recorded thereon, the application field of the disc type information recording medium has gradually tended to expand. A typical disc type information recording medium is a digital video compact disc on which a video signal is recorded by data compressing/restoring processing by means of a method of Moving Picture Experts Group (MPEG). A disc reproducing system for reproducing a video signal read out from a digital video compact disc includes a display equipment such as a color monitor of a television receiver capable of receiving a broadcasting signal as an equipment for outputting the video signal.
When a still screen operation is carried out while the disc reproducing system displays a digital video signal onto a screen of the display equipment thereof, a screen disturbance phenomenon occurs in which a jagged pattern appears at a boundary portion of colors displayed on the screen. The screen disturbance phenomenon is affected by a mutual relation among a chrominance subcarrier, a horizontal synchronizing signal (H.SYNC) and a vertical synchronizing signal (V.SYNC), a method in which luminance/chrominance signal Y/C is separated within the television, and the like, and furthermore relates to a phase of a subcarrier during a horizontal scanning period. The screen disturbance phenomenon of an interlaced scanning system shown in FIG. 1 is distinguished from that of a non-interlaced scanning system shown in FIG. 2. In the interlaced scanning system, the jagged pattern flows at the screen, and an immobile jagged pattern is displayed on the screen in any case of the non-interlaced scanning system. In the interlaced scanning system, as shown in FIG. 1, since the number of scanning lines included in a single field (e.g., an odd field or even field) of a picture displayed on the display equipment is 262.5, a phase of the chrominance subcarrier is inverted at the succeeding field. While closely observing the phase inversion of the chrominance subcarrier and the position of scanning lines, the flowing of the jagged pattern can be recognized.
That is, as is generally known, in case the number of horizontal scanning lines of one field interval is 262 in the non-interlaced scanning system, a period of the chrominance subcarrier included is obtained such that 227.5.times.262=59605 (cycles). Therefore, the phase of the chrominance subcarrier is not inverted at every field but the same to one another, and the state that the jagged color pattern appears at the color boundary portion in the horizontal direction is caused while displaying the still image on the screen of the display equipment, so that picture quality deteriorates.
For example, U.S. Pat. No. 4,933,748 granted to Shinji Katsuki et al. discloses an apparatus for repeatedly reproducing a single field of a color video signal to convert the single field into interlaced fields of a frame signal capable of being displayed by a standard monitor television receiver, in order to solve a phenomenon that the color at the upper portion of a displayed picture is disturbed when a frame signal having the discontinuous phase of its chrominance subcarrier is supplied to the color monitor television receiver.
FIG. 3 is a block diagram for showing an apparatus for processing a repeatedly reproduced single field of a color video signal, described in the above-mentioned U.S. Patent. As shown in FIG. 3, the apparatus for processing the color video signal includes a recording disc 31 having a color video signal of one field recorded in a circular track on disc 31. Such a color video signal of one field is repeatedly reproduced from disc 31 by means of a reproducing head 32 and supplied to a separate circuit 34 from reproducing head 32 through a playback amplifier 33. In separate circuit 34, a luminance signal Y and color or chrominance signal C are separated from the reproduced signal. The separated luminance signal Y is supplied to an FM-demodulator 35 and the resulting modulated luminance signal is supplied to a mixing circuit 36.
The separated chrominance signal C is supplied from separating circuit 34 to an FM-demodulator 37 and the resulting demodulated chrominance signal is fed to a chroma processing circuit 38 in which two color difference signals R-Y and B-Y recorded in a line-sequential manner are made to exit substantially simultaneously. The resulting processed color difference signals R-Y and B-Y are supplied to quadrature two-phase modulators 39a and 39b, respectively. The outputs of modulators 39a and 39b are supplied to mixing circuit 36, and an output of mixing circuit 36 is supplied directly to a contact a of a switching circuit 42 and through a delay circuit 43 to a contact b of switching circuit 42 which further has its movable contact c connected to an output terminal 45.
The chrominance subcarrier signal from an oscillator 40 is supplied both to a 90 degree phase-shifter 41 and to a 180 degree phase-shifter 47. Further, a switching circuit 48 is provided with a switch 48A having fixed contacts a and b connected to the output of oscillator 40 and the output of 90 degree phase-shifter 41, while a movable contact c of switch 48A is connected to quadrature two-phase modulator 39a. A second switch 48B of switching circuit 48 has fixed contacts a and b connected to the output of 90 degree phase-shifter 41 and the output of 180 degree phase-shifter 47, respectively, while a movable contact c of switch 48B is connected to quadrature two-phase modulator 39b. Switches 48A and 48B of switching circuit 48 are operated by a switching signal FS applied to a terminal 44 so that ganged movable contacts c of switches 48A and 48B are changed-over alternately between first and second states or conditions shown in full lines and in broken lines, respectively, at each of the field periods of the color video signal. The chrominance subcarrier produced by oscillator 40 has its phase inverted at every horizontal scanning period in accordance with the NTSC standard.
According to the apparatus for processing the color video signal, even though the chrominance subcarrier provided by oscillator 40 has its phase inverted at every horizontal scanning period in accordance with the NTSC standard, the further shifting of the phase of the chrominance subcarrier by 90 degrees at every field ensures that the phase of the chrominance subcarrier of the frame signal is continuous at the time of the change-over of switching circuit 42 when the field signal issuing from mixing circuit 36 is selectively delayed during alternating fields period for a delay time equal to one-half of horizontal scanning period, as by delay circuit 43 and switching circuit 42, for forming interlaced fields of a frame. Thus, it is possible to obtain a reproduced picture of excellent quality without any disturbance of the color, particularly at the upper portion of the picture.
The apparatus for repeatedly reproducing the single field, however, is applicable only when the single field of the color video signal provided by an electronic still camera scans the color monitor television receiver in the interlaced scanning system in order to be displayed as a still image, thereby being limited in its application.
Consequently, a screen processing apparatus capable of obtaining a still image of excellent picture quality during the still screen operation is required while the picture image is displayed on a display equipment by the non-interlaced scanning system in a video reproducing system such as a digital video reproducing system.